The wave characteristics of the ground formation constitute important basic data for the antiseismic designing of various kinds of buildings and and various kinds of structures formed by civil engineering. In order to investigate the elastic properties of the underground, a PS (P waves and S waves) velocity logging system for directly measuring the propagation of an elastic wave (P wave and S wave) in a bore hole made in the ground is used. The velocity logging is a technique for determining, on the basis of a waveform record, the arrival time of the elastic waves sent out from a wave source to a wave receiver.
Various types of bore hole wave sources have heretofore been developed. Typical examples of bore hole wave sources are disclosed in Japanese Patent Publication Nos. 58-52191/1983, 61-43669/1986, 62-14791/1987 and 62-14792/1987, and U.S. Pat. No. 4,383,591. All of these apparatuses consist basically of a structure provided with a movable member (hammer) which can be moved freely in the direction perpendicular to the axis of a bore hole, and a mechanism for driving the movable member. Such a wave source is set in a water-holding bore hole, and a vibratory force is exerted on a wall surface of the bore hole in a direction perpendicular to the axis of the bore hole by momentarily driving the movable member. The driving mechanism in use usually consists of an electromagnetic solenoid.
However, the conventional electromagnetic hammer type bore hole wave source is driven by an electromagnetic solenoid, and the diameter of the bore hole is comparatively small in general. Therefore, since the outer diameter of the wave source cannot be increased, there is a limit to the magnitude of a vibratory force which can be generated, and the range of application of the wave source is restricted. This wave source is effectively used, for example, in the case where PS wave logging operations are carried out at various depths with a series of probes, in each of which a wave source and a wave receiver are incorporated, inserted in a bore hole without being fixed to a wall surface of the bore hole. However, it is difficult to apply this wave source which has only a limited range of wave propagation to the case where inter-bore hole velocity measurement using two bore holes is conducted.
In recent years, the tomographic techniques for measuring the velocity of elastic waves between two bore holes, and three-dimensionally expressing (tomographically imaging) the wave characteristics over a wide range of underground by using computer-aided data processing techniques have been introduced. According to these techniques, a wave source is set in a wave generating bore hole, one of two bore holes, and multiple connected wave receivers are inserted in a wave receiving bore hole, the other of the two bore holes. The velocity of elastic waves is then measured with the wave source setting position (depth) varied, and the wave generating bore hole and wave receiving bore hole are interchanged with each other. However, when the above-mentioned conventional electromagnetic hammer type bore hole wave source is used, a signal can travel an inter-bore hole distance of only up to 10 m. Consequently, it becomes necessary to make a larger number of bore holes, and the conventional wave source encounters difficulties in practical use.
There is an explosive as a known wave source having a large vibratory force. When an explosive is used, elastic waves reach an object position even if the distance between two bore holes is around several hundred meters. However, the use of an explosive causes the destruction of a bore hole wall. When the measurement of the velocity of elastic waves is conducted by interchanging the wave generating bore hole and wave receiving bore hole with each other, an already-destroyed wave generating bore hole cannot be used as a wave receiving bore hole even if the wave generating point can be shifted in order from a larger depth portion of the bore hole to a smaller depth portion thereof. After all, the necessary measurement cannot be conducted in practice. Moreover, since many limitations are placed on the use of an explosive, it is substantially impossible to use an explosive, especially, in an urban district.